1. Field of the Invention
The present invention is generally in the field of electronics. More particularly, the present invention is in the field of electronic circuit and logic design.
2. Background
Even when an integrated circuit is powered down, i.e. in a “standby mode,” the circuit can consume considerable amount of power, generally known as “leakage power.” For example, an integrated circuit situated in a portable electronic device, such as a cell phone, can rapidly deplete a power source, such as a battery, of the portable electronic device, even during periods when the portable electronic device is in a standby mode. Such power dissipation can be substantially reduced or eliminated by powering down the entire integrated circuit during periods when the integrated circuit is in the standby mode. However, the state of the integrated circuit, i.e. the state data contained in various circuits, registers, flip flops, and memory devices, must be retained prior to powering down the integrated circuit and thereafter restored to the integrated circuit during power up.
One conventional approach for retaining the state of an integrated circuit involves the use of “balloon flip flops,” where each flip flop includes a “shadow latch” (or a “shadow flip flop”) powered by an independent power supply route. Before entering into the standby mode, the state of each flip flop is passed to its shadow latch, while upon entering the active mode (i.e. “power up” mode), the shadow latch writes back the saved value into the flip flop. When entering the standby mode (i.e. the “power down” mode), the supply to the flip flops and to all other circuits can be turned off, while only the supply to the “shadow flip flops” has to be on.
This approach has the advantage that most of the leakage power is reduced or eliminated and the state of the integrated circuit can be quickly interchanged (saved/restored) with that of the shadow latches. However, there are significant implementation complications with this approach, complications that are not easy to overcome. One problem is that the balloon flip flops need two overlapping but independent power supplies, with the associated power routing, one supply to be routed to each flip flop (which will be powered down during standby), and a second supply to be routed to the shadow flip flop (which will remain powered during standby). A second overlapping power supply would have a significant negative impact on the design flow, starting from standard cell libraries, through synthesis and all the way to chip verification. Also, extra routing overhead is required for the signals used to control the save/restore mechanism associated with each flip flop and its shadow. Moreover, the area increase in the “balloon flip flop” approach is significant.